Hair cells are the sensory receptors of both the auditory system and the vestibular system in the ears of all vertebrates.

The site of transduction is in the organ of Corti (spiral organ). It is composed of hair cells held in place above the basilar membrane like flowers projecting up from soil, with their exposed short, hair-like stereocilia contacting or embedded in the tectorial membrane above them. The inner hair cells are the primary auditory receptors and exist in a single row, numbering approximately 3,500. The stereocilia from inner hair cells extend into small dimples on the tectorial membrane’s lower surface. The outer hair cells are arranged in three or four rows. They number approximately 12,000, and they function to fine tune incoming sound waves. The longer stereocilia that project from the outer hair cells actually attach to the tectorial membrane. All of the stereocilia are mechanoreceptors, and when bent by vibrations they respond by opening a gated ion channel (refer to [link]). As a result, the hair cell membrane is depolarized, and a signal is transmitted to the chochlear nerve. Intensity (volume) of sound is determined by how many hair cells at a particular location are stimulated.

The hair cells are arranged on the basilar membrane in an orderly way. The basilar membrane vibrates in different regions, according to the frequency of the sound waves impinging on it. Likewise, the hair cells that lay above it are most sensitive to a specific frequency of sound waves. Hair cells can respond to a small range of similar frequencies, but they require stimulation of greater intensity to fire at frequencies outside of their optimal range. The difference in response frequency between adjacent inner hair cells is about 0.2 percent. Compare that to adjacent piano strings, which are about six percent different. Place theory, which is the model for how biologists think pitch detection works in the human ear, states that high frequency sounds selectively vibrate the basilar membrane of the inner ear near the entrance port (the oval window). Lower frequencies travel farther along the membrane before causing appreciable excitation of the membrane. The basic pitch-determining mechanism is based on the location along the membrane where the hair cells are stimulated. The place theory is the first step toward an understanding of pitch perception. Considering the extreme pitch sensitivity of the human ear, it is thought that there must be some auditory “sharpening” mechanism to enhance the pitch resolution.

When sound waves produce fluid waves inside the cochlea, the basilar membrane flexes, bending the stereocilia that attach to the tectorial membrane. Their bending results in action potentials in the hair cells, and auditory information travels along the neural endings of the bipolar neurons of the hair cells (collectively, the auditory nerve) to the brain. When the hairs bend, they release an excitatory neurotransmitter at a synapse with a sensory neuron, which then conducts action potentials to the central nervous system. The cochlear branch of the vestibulocochlear cranial nerve sends information on hearing. The auditory system is very refined, and there is some modulation or “sharpening” built in. The brain can send signals back to the cochlea, resulting in a change of length in the outer hair cells, sharpening or dampening the hair cells’ response to certain frequencies.

The inner hair cells are most important for conveying auditory information to the brain. About 90 percent of the afferent neurons carry information from inner hair cells, with each hair cell synapsing with 10 or so neurons. Outer hair cells connect to only 10 percent of the afferent neurons, and each afferent neuron innervates many hair cells. The afferent, bipolar neurons that convey auditory information travel from the cochlea to the medulla, through the pons and midbrain in the brainstem, finally reaching the primary auditory cortex in the temporal lobe.

Practice Questions

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Key Points

• The human ear has three distinct functional regions: the outer ear, which collects sound waves; the middle ear, which represents the sound waves as pressure, and the inner ear, which converts those pressure signals into electrical signals that the brain perceives as sound.

• The inner ear exists on the other side of the oval window from the middle ear, by the temple of the human head, and consists of three parts: the semicircular canals, the vestibule, and the cochlea.

• Within the cochlea, the inner hair cells are most important for conveying auditory information to the brain.

• Stereocilia are depolarised by the movement of fluid in the cochlear. The inner hair cells are the primary auditory receptors and the outer hair cells are to fine-tune incoming sound waves.

• Stereocilia are mechanoreceptors, and when bent a signal is transmitted to the cochlear nerve.

• With roughly 90 percent of the afferent neurons carry information from inner hair cells, the inner hair cells are most important for conveying auditory information to the brain.

Key Terms

Transduction: The action or process of converting something and especially energy or a message into another form.

Organ of Corti: A structure in the cochlea of the inner ear which produces nerve impulses in response to sound vibrations.

Hair cells: The sensory receptors in the inner ear that detect sound and head motion to begin the processes of hearing and balance control.

Stereocilia: Small, hairlike protrusions from mechanoreceptors which bend as fluid moves through the cochlea, generating the electrical signals associated with hearing and balance.

Vestibulocochlear cranial nerve: Transmits sound and balance information from the inner ear to the brain.

Cochlea: The complex, spirally coiled, the tapered cavity of the inner ear in which sound vibrations are converted into nerve impulses.